Dear all,

I am a newbie of RooFit, so probably this is a silly question, but I would be sure to face the problem in the correct way.

I am studying a resonance, and then I want to fit its invariant mass spectrum (from which I already subtracted the background) with a Breit-Wigner function.

In this operation, one fundamental quantity I want to estimate is the yield of the resonance, besides its mass and gamma, since I am interested in counting them to make pt distributions and so on.

Now, if I understood well the behaviour of RooFit, when I use a Breit-Wigner PDF, it will be normalized by the RooFit engine before fitting, so I can only estimate the mass and gamma of the peak, but not the yield.

What is the correct procedure to add a multiplicative constant to the normalized Breit-Wigner function (or any other similar peak function, like Gaussian) in order to estimate the yield besides the peak position and width?

Thanks, cheers